1. Field of the Invention
The present invention relates to a lens-fitted photo film unit and an electronic flash device for use therewith. More particularly, the present invention relates to a lens-fitted photo film unit in which an electronic flash device is improved in fashion consistent to structural simplification.
2. Description Related to the Prior Art
A lens-fitted photo film unit is commercially sold so that every one wishing to enjoy photography is enabled to take photographs. The photo film unit includes a film housing, which incorporates a simplified photograph-taking mechanism and pre-loaded with photo film. There are photo film units of various types provided with additional functions: including a model "FUJICOLOR QUICK SNAP FLASH" (trade name; manufactured by Fuji Photo Film Co., Ltd.) incorporating an electronic flash device.
The flash device includes a flash circuit, which, in general has a main capacitor and a neon lamp connected to an end of the main capacitor. When the main capacitor is charged at a predetermined level of voltage, the neon lamp is driven to indicate the standby status of flash emission.
Instead of a neon lamp, it is known in the art to use a light-emitting diode (LED), which is connected within flash circuits illustrated in FIGS. 40 and 41. The LED is less costly than a neon lamp, and can be advantageous for incorporation in the lens-fitted photo film unit in view of a low price at which it should be sold.
FIG. 40 illustrates a conventional flash circuit, where a transistor 640 and a transformer 641 constitutes a self-starting blocking oscillator known in the field of electronic flash. Through a primary winding 641a, a current on the primary side flows, and alternates increases and decreases. This causes a secondary winding 641b to generate an alternate current of high voltage. The alternate current is rectified by a diode 642, to charge a main capacitor 644. The three windings 641a to 641c of the transformer 641 come to have back electromotive force upon change from an increase to a decrease in the current of the primary side. A light-emitting diode (LED) 643 to indicate the standby of flash emission is connected to two ends of the tertiary winding 641c, so that the back electromotive force is applied to the LED 643 in its forward direction.
In course of generation of the back electromotive force, the LED 643 is driven as the potential difference between its cathode and anode becomes greater. As illustrated in FIG. 42A, the blocking oscillator has low frequency in oscillation (as low as 1 kHz) at the start of the charging. A period of absence of the back electromotive force is long, while a period of generation of the back electromotive force is short. Even when the LED 643 is turned on electrically, human eyes may visually find the LED 643 remain turned off. In accordance with increase in voltage of the main capacitor 644, the load to the secondary winding 641b is increased, to heighten the frequency of oscillation of the blocking oscillator. Therefore the interval of successive two times of light emission through the LED 643 becomes shorter and shorter. Apparently the LED 643 emits light quite faintly.
When the voltage across the main capacitor 644 comes to the predetermined level, intervals of the emission of the LED 643 are very short, that is, approximately 10 kHz. The emission appears incessant and bright to human eyes, for indication for the standby. In the flash circuit of FIG. 40, the changes in frequency in the oscillation during the charging are utilized to change the light emission of the LED 643, and adapted to the timely indication for the flashing. FIG. 41 illustrates another conventional flash circuit, which is similar to that of FIG. 40.
In the flash circuit where the LED is driven with the back electromotive force, the faint emission of light from the LED occurs immediately upon the start of the charging of the main capacitor. There is a problem in that a user of the photo film unit may erroneously conceive the standby status of the flashing before the main capacitor comes to have the predetermined level of voltage. As the light emitted from the LED is becoming gradually brighter, there is no conspicuous change even at the time of coming to the predetermined voltage in the main capacitor. It is nearly impossible to indicate the standby status opportunely to the user. Another problem lies in that the LED is caused to emit light in the short period of generation of the back electromotive force. Thus the LED does not emit highly bright light even while the main capacitor has the predetermined level of voltage.
Another problem lies in that part of a current which should come to the tertiary winding flows into a base of the transistor. The current which should flow through the tertiary winding is decreased, which influences the oscillating operation and lengthens time required for charging in unwanted fashion. It would be conceived that an additional resistor might be used for dividing part of voltage of charging the main capacitor, to apply the divided voltage to the LED, which would be driven in response to charging the main capacitor at the predetermined level. However this conception is quite undesirable because the main capacitor would be discharged by way of the resistor or the LED.